This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-276692, filed Sep. 12, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an injection unit of an injection molding machine and, in particular, an injection unit of a three-plate structure.
2. Description of the Related Art
FIG. 3 shows a general arrangement of a conventional injection unit of a three-plate structure. A front plate 5 and rear plate 6 are arranged on a base 1 and connected through a plurality of tie bars 8 to each other. An intermediate plate 7 is arranged between the front plate 5 and the rear plate 6. The intermediate plate 7 is supported by the tie bars 8 and guided by the tie bars 8 such that it can be moved in a forward/backward direction (right/left direction in FIG. 3).
A barrel 2 has a nozzle 3 at its forward end and is connected through the nozzle 3 to a back surface of dies (not shown). A screw 4 is incorporated inside the barrel 2. By rotating the screw 4 inside the barrel 4 a raw resin is introduced into the barrel 2 and, while being heated and kneaded, fed toward the forward end side of the barrel 2. After a predetermined amount of molten resin has been stored in the forward end side of the barrel 2, the molten resin is injected into the dies by forwardly moving the screw 4 in the barrel 2.
The rear end portion of the barrel 2 is fixed to the central area of the front plate 5 and supported there. The rear end portion of the screw 4 is supported at the central area of the intermediate plate 7 through a bearing 11. A servo motor 12 is set over the intermediate plate 7. The servo motor 12 is used to rotate the screw 4 when the raw resin is introduced into the barrel 2.
A ball screw 13 is incorporated in a central area of the rear plate 6. The forward end of the ball screw 13 is fixed to the central area of the rear surface of the intermediate plate 7 through a load cell 14. A servo motor 15 is set on the rear plate 6. In the injection of a molten resin from the barrel 2 into the dies, the motor 15 is used to drive the ball screw 13 and move the screw 4 forwardly.
The conventional injection unit of a three-plate structure had the following problems.
With an increasing size of the screw 4, the forward thrust force of the screw 4 is increased sometimes to nearly a few hundred tons. In the control of the injection pressure of the molten resin, it is usually necessary to detect the forward thrust force of the screw 4. In the detection of the forward thrust force of the screw 4, the load cell 14 is often used as shown in FIG. 3. However, such load cell which detects a load of a few hundred tons is not normal and, upon the manufacture of it, becomes very large in size. It may be considered that a plurality of relatively small sized load cells are arranged in a parallel array to detect a thrust force involved. In this case, a greater space is required to mount these load cells and the injection unit becomes larger in size.
In order to avoid such problems, a proposal has been made to detect a forward thrust force from the strain of an associated component part without using the load cell. In the detection of the forward thrust force from the strain of the rear plate 6 or the intermediate plate 7, the friction resistance of a slide section between the intermediate plate 7 and the tie bars 8 exerts a greater effect on a measured value of a strain involved. It is, therefore, difficult to obtain an adequate accuracy of detection.
In order to drive the screw 4 (and hence the intermediate plate 7) in the forward/backward direction, a ball screw 13 is generally used. If, however, the rigidity of a mounting section of the ball screw 13 is low, a lateral load is generated, thus causing a rapid lowering in the service life of the ball screw 13. It is, therefore, necessary to impart high rigidity to the rear plate 6 and intermediate plate 7 so as to reduce the deflection of these to a smaller extent. If, however, a forward thrust force is to be detected from such small deflection, then resolution is insufficient, thus resulting in less controllability.
Further, the tie bars 8 are also considered as component parts for which it may be possible to detect the strain relatively easily. Even in this case, the friction resistance of the slide section between the intermediate plate 7 and the tie bars 8 exerts some effect on a measured strain value and it is hard to obtain high detection accuracy. Since the tie bars 8 are elongated due to a reaction force induced against the forward thrust force, the rear plate 6 is somewhat moved backward on the base 1 due to small slide movement. This generates a friction resistance and exerts some effect on an elongation amount of the tie bar. It is, therefore, not possible to detect a variation in a very small thrust force.
When the weight of the intermediate plate 7 is increased, then the bending moment acting on the tie bars 8 is increased and, as shown in FIG. 3, a deflection induced in the tie bar 8 cannot be disregarded. Since, in such a case, the distribution of the bending moment varies depending upon the position of the intermediate plate 7, its effect appears on the strain of the tie bar 8, thus lowering the detection accuracy of the forward thrust force.
Further, when the deflection of the tie bar 8 is increased by the weight of the intermediate plate 7, the intermediate plate 7 is tilted and the connected section of a threaded rod of the ball screw 13 is off-centered. Since this off-centered state acts as a lateral load on the ball screw 13, there arises a lowering in the service life of the ball screw 13. In order to decrease the deflection of the tie bar 8, the method for increasing the diameter of the tie bar 8 is considered. However, the elongation due to the forward thrust force is lowered, so that the resolution is inadequate. Further, increasing the diameter of the tie bar 8 is not desirable because this provides a factor for increasing the size of a resultant machine.
Another method may be considered by which, in order to reduce the deflection of the tie bar 8, the length of the tie bar 8 is shortened, that is, the distance between the front plate 5 and the rear plate 6 is made shorter. This method is restricted because the length of the tie bar 8 is restricted by the axial stroke of the screw 4 as well as the mounting space of the ball screw 13.
The present invention is achieved with the above-mentioned problems of the injection unit of the conventional injection molding machine in view and it is the object of the present invention to provide an injection unit of a injection molding machine which can enhance the detection accuracy of a forward thrust force of a screw and obtain a small-sized unit;
According to the present invention there is provided an injection unit of an injection molding machine comprising: a barrel having a nozzle at a forward end and connected to a back surface of a dies through the nozzle; a screw incorporated into the barrel and configured to inject a molten resin into the dies by being forwardly moved in the barrel; a front plate configured to support a rear end portion of the barrel; a rear plate arranged at a backward side of the front plate and connected through tie bars to the front plate; an intermediate plate arranged between the front plate and the rear plate, having through holes through which corresponding tie bars extend, guided by the tie bar to be moved in a forward/backward direction and configured to support the rear end portion of the screw through a bearing; a motor mounted to the intermediate plate and configured to rotationally drive the screw in the barrel; a linear drive device mounted to the rear plate and configured to drive the intermediate plate in the forward/backward direction in the barrel; a fixed base; and a slide table set on the fixed base and configured to support the front plate, rear plate and intermediate plate and movable in the forward/backward direction, wherein the intermediate plate is supported by a liner slide guide on the table to be movable in the forward/backward direction over the slide table.
According to the injection unit of the injection molding machine, the intermediate plate is supported by the linear slide guide on the slide table and it is not necessary to support the weight of the intermediate plate by the tie bars as in the conventional machine. That is, it is only necessary to impart the guide function to the tie bars in the case of moving the intermediate plate in the forward/backward direction (the axial direction of the screw). In this case, a friction force between the intermediate plate and the linear slide guide becomes far smaller than a friction force between the intermediate plate and the tie bars when the weight of the intermediate plate is supported by the tie bars. If, therefore, a load cell is mounted between the back surface of the intermediate plate and the linear drive device and a forward thrust force (injection pressure) of the screw is detected from the output of the load cell, it is possible to obtain a high measurement accuracy.
Further, the deflection of the tie bar is eliminated and a factor which exerts an off-centered effect on the linear drive device is thus eliminated. It is, therefore, possible to prevent a lowering in the service life of the linear drive device resulting from a lateral load involved.
Further, it is preferable that the rear plate be also supported by the linear slide guide on the slide table to allow it to be moved in the forward/backward direction over the slide table.
Since, by doing so, a friction force between the rear plate and the slide table becomes smaller, almost all reaction force induced against the forward thrust force of the screw can be transmitted to the tie bars. As a result, when the forward thrust force (injection pressure) of the screw is detected from an elastic strain of the tie bar, it is possible to enhance its measuring accuracy.
It is to be noted that the linear drive device is comprised of, for example, a ball screw.
Further, in the above-mentioned injection unit, it is possible to omit the slide table. In this case, it is necessary to support not only the intermediate plate and rear plate but also the front plate by the linear slide guide on the fixed base so as to allow these to be moved in the forward/backward direction over the fixed base. If this is so done, a structure of the injection unit can be simplified.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.